1. Field of the Invention
The present invention relates to a method of manufacturing a nickelous positive-electrode active material for alkaline batteries which are used for OA (office automation) equipment, cordless equipment, the power sources of electric vehicles, etc.
2. Description of the Related Art
An alkaline secondary battery is capable of boost charging and high-current discharging. In addition, the alkaline secondary battery is highly immune to overcharging and overdischarging. It has therefore been used for a cordless device, the power source of an electric vehicle, or the like in which a large storage capacity and high charge/discharge efficiencies are required.
In such an alkaline battery, nickel hydroxide (Ni(OH).sub.2) is employed as the active material of a positive electrode. The nickel hydroxide exhibits a high degree of positive-electrode activation in an alkaline aqueous solution because it rapidly generates reversible oxidation/reduction reactions therein as indicated below: ##STR1##
The nickel hydroxide is obtained in such a way that an aqueous solution of a nickelous salt such as nickel nitrate or nickel sulfate and the hydroxide of an alkali metal such as sodium are brought to a neutralizing reaction while the solution is being stirred (Official Gazette of Japanese Patent Application Laid-open No. 2-6340). Further, the electrochemical activity of the nickel hydroxide can be heightened by lowering the temperature and pH of the neutralizing reaction and the concentration of the reacting solution.
The aforementioned prior-art method of manufacture, however, has the problem that nitric acid ions exist as an impurity in the nickel hydroxide thus obtained, so the self-discharging of the battery increases due to shuttle reactions. The shuttle reactions will now be explained in conjunction with the following chemical formulae, by taking as an example a battery whose negative electrode contains a hydrogen-occlusion alloy as its active material: ##STR2##
First, the nitric acid ions existing in the positive-electrode active material are eluted from the positive electrode into the electrolyte solution. Then, as indicated by chemical formula (a), the nitric acid ions (NO.sub.3.sup.-) reacts with the hydrogen-occlusion alloy (MH) contained in the negative-electrode active material, until a product NH.sub.4 OH is formed.
As indicated by chemical formula (b), the formed product NH.sub.4 OH subsequently reacts with a product NiOOH formed in the positive-electrode active material during charging, thereby to form the nitric acid ions (NO.sub.3.sup.-). In this manner, insofar as the battery is in the charged state, the shuttle reactions take place, and the self-discharging proceeds.
With the intention of mitigating such self-discharging of the battery, therefore, a method has been proposed wherein nickel hydroxide formed from a nitrate is washed with water so as to eliminate the nitric acid ions (NO.sub.3.sup.-) (Official Gazette of Japanese Patent Application Laid-open No. 6-223826).
It has also been proposed to dissolve cobalt (Co) for a solid solution into a positive-electrode active material which contains nickel hydroxide (Official Gazette of Japanese Patent Application Laid-open No. 2-109261). Thus, the cobalt is facilitated to accept the high-temperature charge of the nickel hydroxide.
Herein, the solid solution should desirably be of high density in order to ensure a large capacity for the battery per unit volume or unit weight. As the solid solution, accordingly, the hydroxides of nickel (Ni) and cobalt (Co) are not formed from nitrates, but they are formed from the element nickel and the ammine complex Co(NH.sub.3).sub.6 !.sup.2+ (cobalt (II) ion) of the element cobalt.
More specifically, in preparing the solid solution, the solution of sodium hydroxide (an alkali metal hydroxide) is added into a mixed solution consisting of the element nickel and the ammine cobaltous complex. Thus, the solid solution is obtained as one consisting of the hydroxides (Ni(OH).sub.2, Co(OH).sub.2) of the elements nickel and cobalt.
With this method, however, the cobalt (II) ammine complex is oxidized by an oxidant (for example, oxygen) which is contained in a reaction system in the course of the formation of the ammine cobaltous complex, whereby a cobalt (III) ammine complex (an ammine cobaltic complex) is formed as an impurity. The cobalt (III) ammine complex remains without reacting with the alkali metal hydroxide, and it is included in the nickelous positive-electrode active material.
The cobalt (III) ammine complex induces an electrochemical reaction, and forms the product NH.sub.4 OH. This product causes the self-discharging of the battery due to the shuttle reaction indicated by the chemical formula (b). The remaining cobalt (III) ammine complex cannot be eliminated by the aforecited method disclosed in the Official Gazette of Japanese Patent Application Laid-open No. 6-223826.